The present invention relates generally to semiconductor deposition substrates and more particularly to Group III nitride semiconductor deposition substrates and a method for forming same.
In the past Group III nitride semiconductor deposition substrates have been used for laser diodes, modulation doped field effect transistors, ridge wave-guide laser diodes p-n junction type photo-detectors, semiconductor multi-layer reflective films, and sub-band transition devices. Also, high efficiency, short wavelength lasers have been developed using Group III nitride semiconductors containing aluminum (Al), gallium (Ga), indium (In), and nitrogen (N), and having the general structural formula of AlxGa1xe2x88x92xxe2x88x92yInyN (where x and y are mole fractions). However, the wafer size of the single crystalline Group III nitride semiconductors has been small so far because it has been difficult to grow large scale bulk crystals of these materials. In order to attain an acceptable wafer size, Group III nitride semiconductor deposition substrates have been used, which are formed by the deposition of the Group III nitride semiconductors on top of substrates of materials such as sapphire (Al2O3), silicon carbide (SiC), spinel (MgO), gallium arsenide (GaAs), or silicon (Si).
However, this solution has not been entirely satisfactory because there are considerable lattice mismatches and differences in coefficients of thermal expansion between the different types of substrates and the Group III nitride semiconductors. For example, the lattice mismatching is 11% to 23%, and the difference in coefficients of thermal expansion is approximately 2*10xe2x88x926 Kxe2x88x921 between a sapphire substrate and a typical Group III nitride semiconductor. Consequently. Group III nitride semiconductor layers, which consist of a thin film of the Group III nitride semiconductor deposited on different types of substrate, have poor crystal quality, as well as poor electrical and optical properties.
There have been numerous attempts to improve the crystal quality of the Group III nitride semiconductor layers deposited or grown on the various substrates. One of the most effective methods has been to obtain a multi-layer deposition substrate by growing several pairs of single crystalline layers and low-temperature-deposited buffer layers of Group III nitride semiconductor on different types of substrates. Here a low-temperature-deposited buffer layer is one which is deposited at a temperature at which single crystals do not grow.
There are a number of different types of low-temperature-deposited buffer layers, such as those in which single crystallization of the buffer layers is carried out to the desired extent prior to the growth of the next single crystal after deposition.
Occasionally, it is necessary to grow aluminum gallium nitride (AlxGa1xe2x88x92xN, where x is a mole fraction from zero to one (0xe2x89xa6xxe2x89xa61)) on underlying Group III nitride semiconductor deposition layers. A number of different techniques have been developed for accomplishing this.
In Japanese Laid-Open Patent Application No. H 4-297023 to Nakamura, a single crystalline layer of a gallium nitride (GaN)-based semiconductor, grown on a buffer layer of gallium aluminum nitride (GaxAl1xe2x88x92xN, where x is a mole fraction, 0xe2x89xa6xxe2x89xa61) at a temperature at which single crystals will not grow on the substrate, yielded a single crystalline layer of a higher quality, GaN-based semiconductor than when a single crystalline layer of a GaN-based semiconductor was grown on an aluminum nitride (AlN) buffer layer. Nakamura goes on to say that when a GaN thin film was grown on a substrate, the advantages of using a GaN buffer layer, rather than an AlN buffer layer, included:
(1) single crystallization that occurred more readily even when the temperature rose because of the lower melting point (therefore, the benefits of the buffer layer were realized even if the buffer layer was made thicker); and
(2) an increase in crystal quality since the material being grown was the same as the material on which it was grown (e.g., when an epitaxial layer of GaN was grown over the GaN buffer layer).
In Japanese Laid-Open Patent Application No. H 9-199759, Akasaki et al. disclosed a technique in which a low-temperature-deposited buffer layer, formed at a temperature at which single crystals will not grow, and a single crystalline layer, formed at a temperature at which single crystals will grow, were alternately built up in three or more pairs on a substrate of a different material. The targeted Group III nitride semiconductor layer was then formed over the top-most single crystal layer at a temperature at which single crystals grow. A working example was disclosed in which an AlN low-temperature-deposited buffer layer (deposition temperature of 400xc2x0 C. and a thickness of 50 nm) and a GaN single crystalline layer (at a deposition temperature of 1150xc2x0 C. and a film thickness of 300 nm) was alternately built up in three layers each. The uppermost GaN single crystalline layer in Akasaki was grown to a thickness of 1.5 xcexcm and was etched with potassium hydroxide (KOH). The etch pit density was measured from a scanning electron micrograph and found to be 4*107 cmxe2x88x922, after the deposition of one layer pair on a sapphire substrate and 8*105 cmxe2x88x922 when three layer pairs were deposited.
In Japanese Laid-Open Patent Application No. H 7-235692, Sato disclosed a technique for improving the crystal quality of a single crystalline layer grown by using a plurality of low-temperature-deposited buffer layers. A working example was disclosed in which AlN low-temperature-deposited buffer layers and AlGaN low-temperature-deposited buffer layers were consecutively deposited on a sapphire substrate, over which a GaN single crystalline layer was grown to a thickness of 4 xcexcm. The low-temperature-deposited buffer layers were not limited to AlGaN low-temperature-buffer layers and could have been any one of those whose lattice constant was between that of sapphire and GaN.
In Japanese Laid-Open Patent Application No. H 10-256666, Uchida proposed making the deposition temperature of the low-temperature-deposited buffer layers deposited first higher than that of the low-temperature-deposited buffer layers subsequently deposited for improving the crystal quality of a single crystalline layer grown by using a plurality of low-temperature-deposited buffer layers.
In Japanese Laid-Open Patent Application No. H 11-162847, Amano et al. disclosed the basic process of alternately growing multiple layers on the same or different type of substrate of a low-temperature-deposited buffer layer, formed at a temperature at which single crystals will not grow, and a single crystalline layer formed at a temperature at which single crystals will grow.
In Japanese Patent Application No. H 10-313993, Takeuchi et al. disclosed a nitride semiconductor laser element, including a low-temperature-deposited buffer layer containing AlN and a nitride semiconductor single crystalline layer containing AlN and grown directly on the low-temperature-deposited buffer layer as an over 1-xcexcm-thick cladding layer, which provided a single-peak far field pattern.
In Japanese Patent Application No. H 10-322859, Iwaya et al. disclosed that when a GaN single crystalline layer was grown, cracking occurred by the ninth layer pair if a GaN low-temperature-deposited buffer layer was used, but no cracking occurred even with twelve layer pairs if an AlN low-temperature-deposited buffer layer was used. If an AlN low-temperature-deposited buffer layer was used, the in-plane strain of the GaN single crystalline layer was compressive strain and was nearly constant with respect to the number of layer pairs so it was believed that even more layers could be built up without cracking.
Although there has been a great deal of study in this area, there is a need for improvement of the crystal quality of an AlGaN deposition substrate. In particular AlN single crystals are greatly prone to cracking. Using high crystal quality deposition substrates without the cracks means that performances can be enhanced for light emitting and detecting devices that use AlGaN and their yields can also be greatly increased. Further, shorter wavelength (250 to 400 mn) light emitting and detecting elements can be produced by using AlGaN that has a high AlN molar fraction. Even further, AlGaN/GaN multi-layer reflectors (Distributed Bragg Reflectors) with no cracks and extremely high reflectance could be produced.
The present invention provides a multi-layer deposition substrate formed by the deposition of a low-temperature-deposited buffer layer of aluminum gallium nitride (AlGaN) the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction from zero to one (0xe2x89xa6xxe2x89xa61)) on a sapphire or nitride semiconductor substrate, and a single crystalline aluminum gallium nitride (AlGaN) layer, the composition of which is AlyGa1xe2x88x92yN (where y is a mole fraction from above zero to one (0 less than yxe2x89xa61)), which is deposited directly over the low-temperature-deposited AlxGa1xe2x88x92xN buffer layer. The multi-layer deposition substrate is characterized so that x is greater than the greater of 0 and yxe2x88x920.3, and the deposition substrate does not have cracks in this case.
The present invention further provides a multi-layer deposition substrate formed by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0xe2x89xa6yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is characterized so that x is greater than the greater of 0 and yxe2x88x920.2 in order to absolutely prevent cracking.
The present invention further provides a multi-layer deposition substrate formed by the deposition of a low-temperature-deposited AlGaN buffer layer, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is characterized so that y is equal to x in order to obtain better crystal quality of single crystals.
The present invention further provides a multi-layer deposition substrate formed by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is characterized so that x is between yxe2x88x920.1 and yxe2x88x920.3 in order to keep the molar fraction of AlN small and to keep the resistance of the multi-layer substrate relatively low. It is further desirable for x to be between yxe2x88x920.1 and yxe2x88x920.2 in order to prevent cracking.
The present invention further provides a multi-layer deposition substrate formed by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is characterized so that x is equal to yxe2x88x920.2 whereby there will be no cracking and a nitride semiconductor deposition substrate with low resistance will be formed.
The present invention further provides a multi-layer deposition substrate formed by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is characterized so that y is less than l and x is at least 0.05.
The present invention further provides a method for forming a multi-layer deposition substrate by the deposition of a low-temperature-deposited buffer layer of aluminum gallium nitride (AlGaN) the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction from zero to one (0xe2x89xa6xxe2x89xa61)) on a sapphire or nitride semiconductor substrate, and a single crystalline aluminum gallium nitride (AlGaN) layer, the composition of which is AlyGa1xe2x88x92yN (where y is a mole fraction from above zero to one (0 less than yxe2x89xa61)), which is deposited directly over the low-temperature-deposited AlxGa1xe2x88x92xN buffer layer. The multi-layer substrate is formed so that x is greater than the greater of 0 and yxe2x88x920.3, and the deposition substrate does not have cracks in this case.
The present invention further provides a method for forming a multi-layer deposition substrate by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGalxe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is formed so that x is greater than the greater of 0 and yxe2x88x920.2 in order to absolutely prevent cracking.
The present invention further provides a method for forming a multi-layer deposition substrate by the deposition of a low-temperature-deposited AlGaN buffer layer, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is formed so that y is equal to x in order to obtain better crystal quality of single crystals.
The present invention further provides a method for forming a multi-layer deposition substrate by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x92lN (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is formed so that x is between yxe2x88x920.1 and yxe2x88x920.3 in order to keep the molar fraction of AlN small and to keep the resistance of the multi-layer substrate relatively low. It is further desirable for x to be between yxe2x88x920.1 and yxe2x88x920.2 in order to prevent cracking.
The present invention further provides a method for forming a multi-layer deposition substrate by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is formed so that x is equal to yxe2x88x920.2 whereby there will be no cracking and a nitride semiconductor deposition substrate with low resistance will be formed.
The present invention further provides a method for forming a multi-layer deposition substrate by the deposition of a low-temperature-deposited buffer layer of AlGaN, the composition of which is AlxGa1xe2x88x92xN (where x is a mole fraction, 0xe2x89xa6xxe2x89xa61), on a sapphire or nitride semiconductor substrate and a single crystalline layer of AlGaN, the composition of which is AlyGayxe2x88x921N (where y is a mole fraction, 0 less than yxe2x89xa61), which is deposited directly over the low-temperature-deposited buffer layer. The multi-layer deposition substrate is formed so that y is less than 1 and x is at least 0.05.
The above and other advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.